Magnetic Resonance Imaging (MRI) techniques, are rapidly being developed to allow for the quantitative assessment of cerebral blood flow (CBF) in both experimental animals and man. These MRI-based methods offer many advantages over other techniques like inert Xe131-CT, or H2O15 PET, because they are non-invasive and can be acquired registered to high resolution anatomical, functional, and spectroscopic images of cerebral metabolism. These new MRI-based methods of perfusion assessment are based upon the arterial spin labeling technique first introduced by Williams et. al. and include EPISTAR, FAIR, UNFAIR and QUIPPS. These techniques are just beginning to be applied in the clinical setting, and although they are very promising, there is a need to validate these methods and to examine the sources of error in their use. The applicants proposed to systematically investigate the sources of error of these techniques including estimates of transit times and partition coefficients, arterial relaxation times and magnetization transfer effects. They would also compare the estimates of cerebral perfusion obtained with these MR-based techniques with the well established technique of CBF measurement using intracardiac bolus injection of labeled microspheres. These studies would be conducted in nonpathologic normally perfused brain, and under ischemic conditions using middle cerebral artery occlusion in rats. This work could improve the reliability of MRI based measurements of cerebral perfusion and would, therefore, provide for their routine application in clinical and research settings.